The principles of image producing NMR methods, as NMR tomography, are described in an article by I. L. Pykett "NMR Imaging In Medicine" "SCIENTIFIC AMERICAN, May 1982, pp. 54-64. A more detailed explanation of NMR techniques is comprised in a monography by P. Mansfield and P. G. Morris with the title "NMR Imaging in Biomedicine", Academic Press, New York, N.Y., 1982.
In short, an image producing NMR method comprises essentially the steps, to position an object (more precisely a region to be investigated of the object) into a homogeneous, static "longitudinal" magnetic field ("B.sub.o field"), which aligns unpaired nuclear spins within the object into a Z direction parallel to the B.sub.o field, so that a magnetization in Z direction results. This constant, homogeneous B.sub.o field is present in all NMR methods during the entire experiment and this is generally tacitly assumed in the following explanation. Then, the object is irradiated with a radio frequency excitation pulse (RF pulse) by which, from the viewpoint of classical mechanics, the spins are tipped or rotated by a predetermined angle (flip angle) away from the Z direction, the flip angle depending on the amplitude and duration of the RF pulse. The spins will then precess about the Z direction with the so-called Larmor frequency and the transverse component of their magnetization in the plane (XY plane) normal to the Z direction can be detected as output signal. A flip angle of 90 degrees rotates the entire Z magnetization into the XY plane and the output signal has then its maximum amplitude. A flip angle of 180 degrees reverses the direction of the magnetization or spins.
Three relaxation times play an essential role in NMR methods, i.e. the spin-lattice relaxation time T.sub.1, the spin-spin relaxation time T.sub.2 and the effective spin-spin relaxation time T.sub.2.sup.*. Three magnetic field gradients, which are essentially normal to each other and one being generally aligned with the B.sub.o field, are used for coding spatial information into the output signal and for producing specific output signals, such as so-called gradient echo signals. The amplitude and duration of the gradient fields are controlled in accordance with the specific method employed. Further details can be derived from the above-mentioned publications and the patent literature, thus, they are mentioned in the following only as far as they are essential for the present invention.
It has been an aim since the discovery of the NMR tomography to reduce the time ("acquisition time"), which is required for obtaining the NMR data necessary for image reconstruction. Further important features of an image producing NMR method are the spatial resolution (i.e. the dimensions of a resolved image element or pixel of the image which can be reconstructed from the obtained data), and the signal-to-noise ratio (SNR).
A quite rapid NMR tomography method is the Echo Planar Imaging method described by P. Mansfield (J. Magn. Reson. 29, 355-373, 1978), in which a complete free induction decay (FID) signal is detected in the presence of both an alternating magnetic field gradient (in the following in short "gradient") and a constant gradient of predetermined magnitude after a single RF pulse. This method is also described in U.S. Pat. Nos. 4,115,730, and 4,165,479. However, the spatial resolution obtainable with the Echo Planar Imaging method is limited by physical constraints and cannot be improved beyond a certain limited by increasing the measuring time. Also the SNR is not as high as desirable.
Further fast methods for acquiring NMR data for image reconstruction, which use spin echos, are described in U.S. patent application Ser. No. 806,780 filed Dec. 9, 1985, incorporated by reference.